Carrier-wave communicating and switching system



April 6 1926.

E. H. EMYTHE CARRIER WAVE COMMUNICATING AND swncnme SYSTEM Filed Nov. 9, 1923 12 Sheets-She et l April 6, 1926.

E. H. SMYTHE CARRIER WAVE COMMUNICATING AND SWITCHING SYSTEM 12 Sheets-Sheet 2 WE Q QQN V D Filed Nov. 9 1923 A/Aw I April 6 9 1926.

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E. H. SMYTHE CARRIER WAVE COMMUNICATING AND swmcmue SYSTEM Filed Nov. 9, 1923 12 Sheets-Sheet 5 cocoon nmmmN April 6 1926. 1,579,256

E. H. SMYTHE CARRIER WAVE COMMUNICATING AND SWITCHING SYSTEM 12 Sheets-Sheet 6 LJIL Filed Nov. 9 1923 v Mamie/J.

C/mxme/i Mun/745 ZIOOO April 5 1926. I 1,579,256

E. H. SMYTHE CARRIER WAVE COMMUNICATING AND SWITCHING SYSTEM Filed Nov. 9, 1923 12 Sheets-Sheet April- 6 1926. 1,579,256

E. H. SMYTHE v CARRIER WAVE COMMUNICATING AND SWITCHING SYSTEM Filed Nov. 9, 1923 12 Sheets-Sheet 8 I I l 2,4 I

lllll IIIII IIIII lllll l I l 5-9 S-IO. 5-H. LINE-5. NE-4-. |NE4 IIII April 6 1926.

E. H. SMYTHE CARRIER WAVE COMMUNICATING AND swmcnme SYSTEM Filed Nov. 9,1923 12 Sheets-Sheet 9 w w 1 w V A w m Q A] MiG QRQ m mg 5% v Q wg Nwi 35 T h is m N m 3Q w wt Aprifl e 1926, 1,579,256

E. H. SMYTHE CARRIER WAVE COMMUNICATING.AND SWITCHING SYSTEM Filed Nov. 9, 1923 12 Sheets-Sheet 10 April 6 1926. 1,579,256

E. H. SMYTHE CARRIER WAVE COMMUNICATING AND SWITCHING SYSTEM Filed Nov. 9, 1925 12 Sheets-Sheet 11 April 6 1926.

1,579,256 E. HISMYTHE CARRIER WAVE COMMUNICATING AND SWITCHING SYSTEM.

Filed Nov. 9, 1925 12 Sheets-Sheet l2 Patented Apr. 6, 1926.

UNITED STATES N 1,579,256 PATENT OFFICE.

EDWIN H. SMYTHE, 0F EVANSTON, ILLINOIS, ASSIGNOR T0 WESTERN ELECTRIC COM- PAN Y, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CARRIER-WAVE COMMUNICATING AND SWITCHING SYSTEM.

Application filed November s, 1928. Serial No. 678,681.

To all whom it may concern:

Be it known that I, EDWIN H. SMYTIIE, a

citizen of the United States, residing at 4 and more particularly to a system of the type in which carrier Waves or oscillations, either. free or guided, are employed as the medium for conveying energy for effecting the operation and control of the electrical switching, signaling and communicating apparatus located at separated points in the system. The invention especially relates to a system in which carrier currents in the form of high frequency waves guided upon wires are used as the medium of electrical connection between the electrical devices and I apparatuses located at different points in the system. These-carrier currents are assigned such frequencies that they are noninterfering with one another, so that each carrienserves as a connecting medium separate and distinct from the other carriers superposed upon the line or medium over which the carriers are transmitted. Each each of which may have associated with itapparatus that serves to reproduce from a modulated carrier the voice, signal or controlling currents with which the carrier was modulated at the distant station. The arrangement which is used to transmit and receive by means of any one high frequency wave as distinguished from that which is used for transmitting and receiving by" means of a wave of a different frequency is commonly known as a high frequency or carrier channel, or as a frequency selective channel.

The present'invention is directed particularly toward providing a system in which a connection may be established between two or more toll or long distance stations, through an intervening switching station or toll center .and the carrier multiplex lines that connect the long distance stations with the toll center, without the necessity of employing manually controlled switching or interconnecting agencies at,

the intermediate station; and toward providing a carrier multiplex interconnecting or switching system in which the switching or interconnecting of any chanml of any calling line with any available channel of any desired other line may be effected merely by the shifting of the carrier frequencies at the intermediate station; and without the inclusion of any movable switch or relay contacts of any kind in the voice modulated carrier circuits. In the preferred embodiment of the invention herein disclosed, all of the transmitting and receiving branches of all of the channels of all of the carrier multiplex lines that center in the switching station are'permanently connected together by way of frequenc selective paths that normally prevent any 0 annel from being in operative relation with any other channel; but which are able, when the proper frequency shifting operations occur under the control of the operators at the distant toll stations, to afford operative connection in any combination desired betweenthe channels of the various lines.

The resent invention has for .its principal o jects the provision of a carrier multiplex communicating system in which station selection is accomplished by shiftin the carrier frequency; in which the shifting of the carrier frequency is employed for selecting a particular carrier multiplex line and also for selecting a particular available channel of the selected I line; in which the line selecting shift of.

the carrier frequency is through a relatively slight range and is accomplished by the application of modulating frequencies of a frequency order that permits the ready suppression of undesired products. of modulation; in which the modulating frequencies are applied and withdrawn locally in the intermediate or switching station and are not permitted to pass into the carrier multiplex lines extending to the distant toll stations; in which the carrier shifting frequencies are so chosen as to be relatively low in the frequency scale to permit proper shielding and prevent undue radiation, while at the same time being high enough in the frequency-scale to permit the ready suppression of undesired modulation products; in which the successive steps of carrier modulation to effect the desired frequency shift is accomplished with a minimum of modulating apparatus; in which the application of the modulating frequency at the switching station to shift the carrier frequency to select any desired line is ac-' complished by switching mechanism under the control of the operator at the calling station; in which said switching mechanism operates automatically to select an available channel extending to the called station; in which said switching mechanism acts automatically at the end of its line and. channel selecting operation to adapt the incoming carrier frequencyofthe calling line to the outgoing channel of the called line, and the incoming carrier frequency of the called line to the outgoing channel frequency of the calling line; in which the incoming channel branches are permanently connected with and converge upon a common outlet from which frequency selective branches diverge to the outgoing channel branches of the various carrier multiplex lines; and in which the incoming channel branches are connected with the common outlet through the medium of a conjugate network that insures the uni-directional flow of the voice modulated carriers into the common outlet circuit and prevents the feeding back of the voice modulated carrier of any channel into the incoming paths of the other channels connected with the common outlet.

The foregoing are some of the principal objects that the system of the present in vention attains. These and other features are all included in the present embodiment of the invention, but are capable of separate use in systems where it may be desirable to employ only part of the features herein disclosed. The manner in which the above mentioned and other objects and features" are attained and. realized will be explained in the following description of the preferred form of the invention, and the structures and combinations involved in the realization of the various novel features will be more particularly defined in the appended claims.

The preferred embodiment of the invention is illustrated in the accompanying drawings, in which Figs. 1 to 10, inclusive, taken together and arranged in the manner indicated in Fig. 11, show diagrammatically as much of the apparatus and circuits of the intermediate switching station and of one of the toll stations as is necessary to disclose the manner in which the apparatus and circuits of the entire system may be organized;

in which Fig. 12 shows a somewhat different arrangement of the line selection modulating circuit; and in which Fig. 13 shows a modified arrangement for preventing feedback between the incoming channel branches connected with the common outlet. More specifically, Fig. 1 illustrates the local line and carrier multiplex line terminals at one of the toll stations and the operators cord circuits for interconnecting the terminals, together with apparatus for producing the required carrier and controlling frequencies; Fig. 2 illustrates the associated channel apparatus at the toll station; Fig. 3 shows the apparatus at the intermediate or switching station for producing the demodulating channel matching and station selecting frequencies, and for modulating certain of the frequencies with a busy tone in connection with the production and transmission of a return busy test signal; Fig. 4 shows the switching station channel apparatus associated with the carrier multiplex line extending to station 1, the organization of channel 1 being shown in full and that of channels 2 and 3 being indicated in outline; Fig. 5 shows in skeleton the switching station channel apparatus associated with the lines extending to station 2 and station 3; Fig. 6 shows the switchingstation channel apparatus associated with the line extending to station 4, the organization of channel 1 being shown in full and that of channels 2 and 3 being shown in skeleton form; Figs. 7 and 7 illustrate diagrammatically the stepby-step switches and their circuits and the manner of their association with the twelve channels of the four carrier multiplex lines, which switches are controlled by the operators of the respective lines and in turn control the application of the channel matching and station selecting modulating frequencies; and Figs. 8, 9 and 10 illustrate the conjugate network connecting the incoming or receiving branches of the twelve channels with their common outlet circuit, and the frequency selective branches and frequency reducing apparatus that connect the common outlet with the transmitting or outgoin branches of the different channels of the our carrier multiplex lines.

Reference will first be made briefly to the general organization of the system as shown in the drawings, and then a more detailed description of the system will be 1 given. In the preferred embodiment of the invention illustrated there are four carrier multiplex lines centering in an intermediate or switching station. These lines extend from station 1, station 2, station 3 and station 4, and each may be organized as illustrated in Figs. 1 and 2. At each of the toll stations local lines terminate, and provision is madefor establishing connection between these local lines and the various channels of the carrier multiplex line through the medium of operators connecting cord circuits. The channel terminals, the local line terminals and the operators connecting cord circuits all have associated with them appropriate calling and supervisory signals, and the cord circuits are equipped with switches and keys for controlling the connection establishing and disestablishing operations. line extends to the intermediate or switching station; and, as shown in the present embodiment of the invention, each of these toll lines is provided with apparatus to constitut/e three non-interfering selective channels, which in each case are designated channel 1, channel 2 and channel 3. In the case of all of the toll lines entering the switching station from stations 1, 2, 3 and 4, channel 1 at the switching station in each case is arranged to transmit uponthe upper side band of the 10,000 cycle carrier wave and to receive upon the lower side band of the 23,333 cycle carrier wave; channel 2 is ar-, ranged to transmit upon the upper side band of the 13,333 cycle carrier and to receive upon the lower side band of the 26,666 carrier; and channel 3 is arranged to transmit upon the upper side band of the 16,666 cycle carrier and to receive upon the lower side band of the 30,000 cycle carrier. At the distant end of each of the toll lines in the present embodiment. of the invention, the frequency selective construction and adjustment of the channel apparatus is such that reception is effected upon the same frequency band which is used for transmission atthe switching station while transmission at the distant station is effected over the same frequency band that is employed for reception at the switching station. That is, at each of the distant or terminal stations channel 1 receives over the upper side band of the 10,000cycle carrier and transmits over the lower side band of the 23,333 cycle carrier, channel 2 receives over the upper side band of the 13,333 carrier and transmits over the lower side band, of the 26,666 carrier,

and channel 3 receives over the upper side band of the 16,666 carrier and transmits over the lower side of the 30,000 carrier.

At the intermediate or switching station, as represented in Figs. 3 to 10, inclusive, means are provided for bringing any channel of any one of the toll lines into communicating relation with any channel of any of the other three tool lines entering the switching. station. All of the channels of all of. Y the stations are permanently connected together at the switching station through the medium" of an interconnecting network in which are interposed frequency selective devices that permit the establishment-of op'erative relation between any two channels only when certain appropriate frequency changes F rom each terminal station a toll through the agency of frequency shifting apparatus including in the receiving or incoming branch of each channel and controlled by the operators at the distant stations through the medium of the switches shown in Figs-7 and 7 of the drawing. As indicated there are twelve of these switches, onefor each of the three channels of the four toll lines centering in the intermediate or switching station. Each of these switches is capable of being moved selectively into any one of a number of different positions; and in each of its selective positions it controls the energization of relays associated with the calling channel identified with the switch itself and the called channel corresponding to the position to which the switch is moved which relays operate to apply the appropriate modulating frequencies to the frequency shifting apparatus of the two channels; The carrier frequency of the incoming branch of the channel of each line is thus shifted to render it capable of passing through the frequency selective branch to the other line and to the outgoing branch of the proper channel. of 5 that line. The modulating frequencies for effecting the line. selecting and channel matching shifting of the-carriers are produced from a fundamental or base frequency by means of the harmonic generating apparatus illustrated in Fig. 3 of the drawing. The base frequency is applied to all of the carriermultiplex lines centering in the switching station and is thus made available at. each of the outlying or toll stations for the harmonic production of identical modulating and demodulating frequencies at all points in the system.

Referring now more in detail to theorganizationof the system as illustrated in the drawing, the terminal toll station illus trated in Figs. 1 and 2, which may be re-' lines of an ordinary telephone exchange.

These" local lines arerepresented as being of the central ener type and as extendin from the local su scribers stations, as S byway-of line circuits 101, 102 and 103 to terminal apparatus at thecent'ral ofiice. The terminal apparatus ofeach line is represented ascomprising the usualcut-ofi relay 10, line rela '11 and line lamp LL; and

each line is rther assumed to be provided with an answering spring jack AJ and one or more multiple spring jacks LMJ. The

circuit-arrangements of such'line's'are well known in the art and-need not be s-peci-.

e 1915, to O. F.'Forsberg.

sired type of line for establishing communication between the central office and the subscribers stations may be substituted for the type of line particularly illustrated.

At the central ofiice or toll station two operators positions are indicated, and there may be as many operators positions provided as are required to handle the local and long distance or toll traffic. Each of the two operators positions illustrated is shown to be provided with a plurality of connecting cord circuits, one connecting circuit at each position being shown substantiall complete and another merely indicate by branches from the common leads. Each of the connecting circuits is illustrated as terminating in a local plug LP and a toll plug TP the tip and ring contacts of which,

are united by means of a pair of talking conductors with an interposed repeating coil RC. Each connecting cord circuit has a local supervisory lamp LSL controlled by the connected local line through the medium of the supervisory relay 12, and a toll supervisory lamp TSL that is under control of the channel with which the associated toll plug TP is brought into connection. Each connecting cord circuit also is provided with a ringing key RK, a listening key 'LK and a calling key CK. These keys are respectively for the purpose of signaling the local subscribers stations and bringing the operators telephone set OTS and the operators dial OD, common to the cord circuits of each position, into operative relation with the particular cord circuit with which the listening key LK and the calling key CK are associated, The operators dial OD may be of the well known type disclosed in Patent 1,161,854, issued November 30th, On account of certain speed limiting factors in the circuits controlled by the dial in the present em- ,bodiment of the invention, as will appear in the detailed description that follows, the speed governingmechanism of the dial is adjusted so as to retard the rate at which the dial in it operation opens and closes the circuit that it controls.

The carrier multiplex toll line through which th terminal station has communication with the other stations ofthe carrier multiplex system is generally of the. type disclosed in Fig. 49' in the Colpitts and Blackwell paper entitled, Carrier current telephony and telegraphy, published in the Journal'of the American Institute of Electrical Engineers for April, -May andJune, 1921. Referring to Fig. 2 of the application drawing, the toll line comprises a .pair of metallic conductors 104: that enter the ter-' minal station by way of a high frequency hybrid coil HFH 1 provided with the usual network N for balancing the toll line and V producinga conjugaterelation between theators position -1 and windings of the coil. From the hybrid coil there is connection by way of the apparatus of channel 1, channel 2 and channel 3 to springj ack terminals associated with each of the three channels, these being designated respectively TJ.1, TJ2 and TJ8 at oper- MJ1, MJ2 and MJ3 at operators position 2. All of the channel springjacks at the two positions have associated with them busy lamps, designated BL1, BL''2 and BL-3, respectively, for the three channels; and each of the toll line terminal springjacksaat operators position 1 has also associated with it a calling lamp designated CL 1, CL-2 and CL-S, respectively, for the three channels. The busy lamps, as will be explained, are lighted whenever the corresponding channel is in use for-either an outgoing or incoming con.- nection, and the calling lamps CL are lighted by incoming calls over their respective channels.

Associated with each channel there are two relays 15 and 16 responsive to the insertion of a plug in one of the corresponding channel sprin jacks and controlling the energization of the carrier channel and the application to the carrier of the low controlling frequency; a relay 17 that responds to the condition of use of the channel and causes the lighting of the associated busy lamps; a relay 18 that responds to the application of the calling frequency at the distant end of the channel and controls the calling, supervisory and busy lamps; ant two relays 19' and 20 that operate under the control of the other relays to energize the transmitting branch of the channel and zip ply to it the carrier frequency and the low calling and controlling frequency. The high frequency hybrid coil HFH1 of the toll line has a main transmittin branch and a main receiving branch, and from each of these main branches there is a sub-branch to each of the three channels. Interposed in each of these sub-branches is a band filter, BF1, BF-Z, BF3,'BF4, BF5 and BF-6, respectively, so constructed and adjusted as to pass the frequency bands comprising in each case the appropriate sideband of the modulated carrier wave of the frequency respectivelyindicated by the numerals associated with the rectangles representing these filters, the frequency bands for the various'channels being in accordance with what has already been stated in connection with the general description of the referred to. The organization of only one of the three channels, channel 1, is illustrated in Fig.2. The other two channels are organized in the same manner as channel 1 excepting in the -matter of the specific fre-- queney values on which they are designed to operate, and are merely indicated by broken line rectangles. As specifically illustrated in the case of channel 1, the frequency band that passes through the band filter BF2 included in the receiving branch is carried into the balanced detector BD-l where it is joined by the corresponding carrier frequency selected from the common distribut-' ing circuit 105 by the tuned circuit TC'J=. Demodulation takes place in the balanced detector BD1, and the products of demodulation are amplified in the amplifier A1 and are carried to the low pass filter LPF1 and theband filter BF8. The low pass filter LPF1, in the present instance, is

' constructed and adjusted to have an upper cut-01f limit of 200 cycles per second and the band filter BF8 is constructed and adjusted to pass with substantially negligible attenuation a frequency band extending preferably from 300 to 2300 cycles per second. The relatively low calling and controlling frequency conveyed by the carrier passes through the low pass filter LPF1, is rectified and amplified in the rectifier R-land energizes therelay 18. The essential voice frequencies pass through the band filter BF-8 and are carried to the mid-points of the line windings of the low frequency hybrid L-FH-1. One end of the line windings of the low frequency hybrid coil is connected throt gh condensers with a low frequency balancing network N and the other end is connected through condensers and through an artificial line AL-1 of the wellknown H type by way of conductors and 51 to the tip and ring or talking springs of springjack TJ1, and also to the corresponding springs of such other multiple springjacks MJ1 as'may be associated with this particular channel.

' The series winding of the low frequency .hybrid LFH-l is connected by way ofthe band filter BF7 with the primary Winding of the transformer '21. The band filter BF-7 is of the Campbell type previously referred to, and is constructed and adjusted to pass a band of frequencies ranging from 300 to 2300 cycles per secend and to cut off frequencies below and above these limits. The outer terminals of the secondary winding of the transformer 21 are connected respectively with the grids of the two thermionic vacuum tubes of the balanced modulator BM1, and the inner ends of the two halves of the secondary winding of the transformer are connected with movablecontact points of the potentiometer 22, the mid-point of which is connected by way of the C batploy for transmitting purposes, in the pre'sent instance of frequency of 23,333 cycles per second. The organization of the modulator BMI is in accordance with the disclosure of Patent 1,343,306, issued June 15, 1920, to J. R. Carson, the modulator being of what is commonly known as the balanced or carrier suppression type. Thatis, the effect of the potential variations in the divided input circuit of the modulator is balanced and neutralized in the output circuit excepting when thelow frequency voice current fluctuations are impressed by way of the band filter BF7 and the transformer 21 or when the low frequency calling or controlling current variations are impressed by way of the potentiometer 22. In either of these cases the fluctuations in the output 'circuit of'the modulator are no longer balanced and consequently oscillations are produced in the output circuit having an amplitude proportional to the instantaneous value of the low frequency speech or low frequency controlling modulating currents. The movable points of the. potentiometer 22 are so placed that the amplitude of the voltages impressed upon the gridsby the low frequency controlling current are properly p roportionedto the am plitudes of, the other voltages impressed upon the grids, and also so that the voltages impressed upon the grids by the carrier current flowing in the potentiometer 23 are of equal value. p

The 150 cycle control frequency is derived from the common distributing circuit 106 by way of a tuned circuit TC-2, the. connec-' tion with'the primary side of the tuned circuit being controlled by normally open contacts of the relay 19, and the secondary side of the tuned circuit being connected with the extremities of the potentiometer 22. The

tuned circuit acts not only to insure the application to the balanced modulator BM1 of the pure control frequency, but also to prevent the backing up into the common distributing circuit 106 0f the other frequencies that are present in each of the balanced -modulator circuits, namely, the car- 1 rier frequency of each of the various channels and the voice frequencies with which the carrier is to be modulated. The carrier wave employed in the transmitting branch of the channel is derived from the common distributing circuit 105 by way of a tuned circuit TC-3 which is constructed and adjusted to be selective of the particular carrier frequency that the channel employs. The connection with the primary side of the tuned circuit TC3 is controlled by normally open contacts of the relay 20, and the secondary side of the tuned circuit has connection with the extremities of the potentiometer 23.

The output side of the balanced modulator BM-1 is connected by way of the windings of the transformer 24 and a high pass filter HPF1 with the input side of the balanced amplifier BA1. The high pass filter HPF1 acts to prevent the passage of the amplified and unmodulated low con trolling and voice frequencies to the balanced amplifier BA-l in order to prevent these frequencies from overloading3 the amplifier. The balanced amplifier A-1 is of the so-called push-pull type as illustrated and described in connection with Fig. 49 of the Colpitts and Blackwell paper hereinbefore referred to, and the high pass filter HPF-1 is of the sort also illustrated and described in the Colpitts and Blackwell paper. The output of the balanced amplifier BA1 is connected by way of the band filter BF-1 with the main transmitting branch that joins the various channel transmitting branches with the high frequency hybrid coil HFH-l.

As the tubes of the balanced modulator BM1 and the balanced amplifier BA-l are required to be active only when the associated channel is in use, the circuit of the A battery for heating the filaments of the tubes is controlled in contacts of the relay 20 which are normally open and are closed only when the relay 20 is energized by the taking of the channel for use. The circuit of the A battery is illustrated as extendin in par-- allel branches to the filaments of theTmlanced, modulator and amplifier tubes; but it will be understood that this is merely a diagrammatic showing, and that the energizing circuits of the filaments of the tubes may be connected with each other and the associated circuits in accordance with the standard practice of the wiring of these devices. In-

asmuch as it is desirable that the receiving branches of the various channels be normally in a responsive condition in order that they may constantly be in readiness to respond to' a call originated at a distant station, it is assumed that the thermionic tubes associated with the receiving branches have their filaments normally energized, and therefore the A battery circuitfor these tubes and means for controlling it are not shown.

The apparatus by means of which the carrier frequencies for use in the various channels are produced'is shown diagrammatical- 1y atthe bottom of Fig. 1 of the drawing.-

also has another branch that has interposed in it ,a tuned circuit STC-1 which is tuned to permit the passage through it of the base frequency of 3,333 cycles per second supplied over the carrier multiplex toll line from one of the remote stations, as will be described, and also includes a shunt made anti-resonant to the 3,333 frequency and which therefore permits the substantially unattenuated passage of the other frequencies in the series circuit of the high frequency hybrid coil. receiving branch. The tuned circuit STC1 is connected by way of conductors 107 with the input circuit of the amplifier A2 (Fig. 3) and the output circuit of the amplifier is connected with the input circuit of harmonic generator HG1. The output circuit of the harmonic generator is connected with the input circuit of the amplifier A3 and the output circuit of amplifier A-3 is connected with the dis tributing circuit 105. This train of apparatus acts to generate and amplify a series of harmonics of the base frequency 3,333 in the manner described in the Colpitts and Blackwell paper previously referred to, the nature and arrangement of the apparatus being in accordance with the disclosure contained in U. S. patent to Burton 1V. Kendall 1,446,752 dated February 27 1923.

The low control frequency, 150 cycles per second in the present embodiment of the invention, is produced in the oscillator O1 (Fig. 1), which oscillator may be of any usual and well known thermionic type with a feed back connection from its output into into its input circuit. This sinusoidal oscillations produced in the oscillator O-1 are amplified in the amplifier A1 and thence pass through the tuned circuit TC-l, which serves to suppress the undesired frequencies, to the primary winding of the transformer 14. This transformer is provided with a grounded shield between the primary and secondary windings to prevent unbalance to ground through the transformer of the two sides of the distributing circuit 106 which is connected with the secondary winding of the transformer. The distributing circuit 106, as has been-described, has branch connections with the different channels through which the 150 cycle control frequency is supplied for the modulation of the channel transmitting branch carriers under the control of relays such as relay 19 of channel 1.

The organization of the circuits and apparatus at the intermediate or switching station will now be described by reference to Figs. 3 to 10, inclusive, of the drawing. Fig. 4 shows in detail the organization at the switching station of channel 1 of the carrier multiplex line extending to station 1 and Fig. 6 shows in detail the organization at the switching station of channel 1 of the carrier multiplex line extending to station 4, only enough of ,the organization of the other two channels of each of these lines being shown to illustrate the connection of the carrier shifting modulating frequencies to the channel apparatus. In like manner Fig. shows only-enou h of each of the three channels of each 0 the carrier multiplex lines extending to station 2 and station 3 to illustrate the connection of the carrier shifting modulating frequencies with these channels. Except in the matter of the particular carrier shifting frequencies that are made available for use in connection with each of the channels, the channels are all identical in their organization. Channel 1 of the line extending to station'l andchan nel 1 of the line extending to station 4 are shown in full in order that a connection established between two stations may more zation of the carrier multiplex line illustrated in Fig. 4 and of channel 1 of'this line will suflice for all of them. I

- The conductors 104 of the toll lineare connected with the main receiving branch 125 and the main transmitting --branch 126 by means of the group filters GF1 andv GF.2 respectively. The group filters 'GF.-land GF-2 are of the type disclosed in the Campbell patent previously referred to. The filter GF-1 isconstructed and adjusted to pass a frequency band extending from 21,000 to 30,000 cycles and the filter GF'2 is constructed and adjusted to pass a frequency band extending from 3,000 cyclesto 19,000 cycles. Each group filter excludesthe frequencies that the other group filter (shown in Fig.4 may be used at. both ends pf passes, thereby preventing any local communication of energy from-the main transmitting branclTto the main receivin branch by Way 'ofthe group filters. If des1red.this result may be accom lished by the use of a high frequency hybrid coil with a balancing network, such as is shown in association with the other endof the -carrier multiplex line in-Fi 2; or separating group filters as the line:

The; main receiving branch conductors 125 are connected by way of band ,filters TEE- 9, BFr-dl and BF13 with-there- "f-wceiving branches of channels 1, 2 and}; rt,-

spec tivelyj and the .main ".transnnttmg branch conductors 126have connection by way f bani-filters BEE-1.0, --B F 12 are -modulator BM-2.

frequencies ap- 'BF.14 with the transmittingbranches ofchannels 1, 2 and 3 respectively, The main,

transmitting branch. 126 also is connected through the medium ofthe band filter BF- 19 with the apparatus illustrated in Fig. .3

that generates the base frequency of 3,333

cycles per second, All of these band filters areof the type. disclosed -in the Campbell patent previously referred to.

Referring now to the typical channel cir.- cuit arrangement illustrated in connection with channel 1 of Fig. 4, the receiving branch extends from the band filter BF,.-')

to the series input circuit of the balanced The balanced modulator BM-2 is of the type disclosed in Patent 1,343,306 to Carson previously rosff ferred to, in which the currents to be modulated are appliedserially to the input circuit of the two. thermionic tubes ofthe modulator and the modulating frequencies are applied by way. of the common portion" of the divided input circuit of the two I tubes. In the case of channel 1. the currents that -are-commum'cated to the series input circuit of the balanced modulator BM'2 are the currents comprised in thelower sideband of the 23,333 cycle'carrier that passes.

through the receiving branch band filter BF9 and the modulating currents supplied to thethermion'ic. tubes of balanced I modulator BM2 in parallel arethe harmonic' modulating frequency of 23,333 cycles, corresponding 'tothe carrier frequency of this particular receiving branch,

and one or another of the harmonic modulating frequencies of 33,333 cycles, 36,666

1 cycles or 40,000 cycles, depending uponthe particular called line channel the frequency ofwhich is to be-matched. The 23,333 cycle demodulating' frequency and the particulari desired one of the-other three channel matching or carrier shifting frequencies are supplied to the parallel input circuit of the balanced modulator 'BM-2 through the medium ofthe high frequency hybrid co1l' -HFH'2. .One end. of'the line, windings of the high frequency hybrid coil, is connected with the parallelinput circuit of the balanced modulator and the other end of the line windings is connected with a balancingnet or impedance N the value of which is so chosen as to balance theimpedance of the input circuit of the balanced modulator. The mid-points .of the high v frequency hybrid coil HFH-2 are-com nected with the 23,333 cycle distributing circuit 109, and the series winding of the high frequencyhybrid coil isfconnected with.

a circuit having branches extending to.

normally open front contacts of the relays 61, 62 and 63. :These three relayshavetheir armatures connectedrespect vely with the 33,333 cycle distributing C1IC111i3 112,1the

gswfi'vcychdistributing circuit 3 and we 3 T introduced into the associated distributing circuits when the relays 61, 62 or 63 are operated to connect with their corresponding distributing circuits with the series windings of the high frequency hybrid coil HFH-2. This is in order that the impedance of each distributing circuit, lookingout from the associated band filter of Fig. 3,;may remain substantially constant regardless of the condition of use or dis- 'is actuated to close its front contacts.

use of the various distributing circuit branches. The. interposition of the high frequency hybrid coil HFH2 in the circuit between the parallel input circuit of the balanced modulator BM2 and the two harmonic frequency modulating branches that are adapted simultaneously to feed modulating high frequency currents to the balanced modulator, maintains a conjugate relationship between the two branches so that while each branch feeds its particular frequency into the input circuit of the *balanced modulator neither branch communicates its frequency in any substantial amount tofthe other branch.

The modulating'action which takes place in the balanced modulator BM2 may now be briefly considered. The lower side band of the 23,333 cycle carrier that is applied to the series input circuit of the balanced modulator BM2 is modulated by the 23,333 cycle harmonic frequency and also one or another of the three harmonic frequencies 33,333, 36,666 or 40,000, depending upen which of the relays 61, 62 or 63 sidering first the modulation of the lower side band of the 23,333 cycle carrier by the 23,333 cycle harmonic frequency supplied through the' hybrid coil HFH-2, the products of modulation which appear in the output circuit of the balanced modulator are primarily the amplified 23,333 cycle side band and the sum and difference products of modulation of this side band with the 23,333 cycle harmonic frequency. The sum product of modulation is the lower side band of the 46,666 cycle frequency. The difference product of modulation is the relatively low frequency with which the 23,333

cycle carrier was modulated at the distant toll "station to produce the side band that is passing through the band filter BF- 9. In the output circuit of the balanced modu lator BM-2 are the Campbell type band filter BF-15 and low pass filter LPF2, filter BF-15 being constructed andadjusted to pass a band of frequencies ranging from 10,000 to 19,000 cycles and the filter LPF-2 being constructed and adjusted to pass a band of frequencies ranging from 0 to 150 cycles inclusive. ,Therefore, the controlling frequenc of 150 cycles with which the carrier uring the use of the channel is modulated at the distant toll station, is reproduced by demodulation in the balanced modulator BM2 and flows from the modulator output circuit by way of the low pass filter LPF2i All of the other products of modulation produced in the balanced modulator BM2 by the modulation of the 23,333 cycle lower side band with the 23,333 cycle harmonic frequency are above and outside the range of both the filters BF15 and LPF2 connected with the modulator output circuit. Furthermore, the speech frequencies produced by this modulating action are limited to a range between 300 c cles and 2300 cycles, and are therefore above the upper cut-off limit of low pass filter LPF2 and are below the lower cut-off limit of band filter BF-l5. Therefore, the' only frequencies resulting from modulation by the 23,333 cycle frequency that can pass from the output circuit of the modulator BM -2 is the 150 cycle controlling frequency.

Substantially the same action as that described above occurs in each of the switching station receiving channel branches of all of thegcarriermultiplex lines when the corresponding channels are in use, as each channel has constantly applied 'to the parallel input circuit of its associated balanced modulator BM2, the harmonic demodulating frequency corresponding to the carrier frequency of that channel. Therefore, in each case, the application of the 150 cycle controlling frequency at the toll terminal station of the channel results in the reproduction of that frequency in the output circuit of theassociated balanced modulator BM-2 at the switching station, and the passage of the reproduced 150 cycle controlling frequency through the associated low pass filter LPF2. 7

After passing through the low pass filter, the reproduced 150 cycle controlling frequency is amplified in the amplifier A-5, and after rectification in the rectifier R2 is effective to control the operation of the relay 25 for purposes that will hereinafter appear.

Returning now to a consideration of the action that occurs in balanced modulator BM'2 as a result of the modulation of the 23,333 cycle carrier lower side band by the harmonic frequencies applied under the control of relays 61, 62 and 63, it may be said in the first place that if the modulator carrier of channel, 1 is to be matched with the frequency of channel 1 of another line, relay 61 is operated to apply the 33,333 cycle harmomc i equency; if the modulated car- 

